The present invention relates to a method for monitoring and optionally controlling the absolute inventory of a fluid, especially a polymer, in a vessel and more particularly in such vessels having at least one agitator with a rotatable shaft extending in an essentially horizontal direction. The method of the present invention involves measuring a resultant force acting on the agitator in an essentially vertical direction in relation to the shaft(s) (i.e. radial to the shaft), relating such resultant force to an absolute inventory value, and optionally controlling the absolute inventory of fluid based upon the inventory value. The present invention also relates to a device which measures a resultant force acting on the agitator in an essentially vertical direction in relation to the shaft(s), which resultant force may be related to an absolute inventory value.
Various methods have been proposed for measuring properties of fluids. Such methods include, for example, methods for measuring the viscosity, elasticity, and flow rate of a fluid. U.S. Pat. No. 4,468,953, Garritano, discloses a torsion tube apparatus for making on-line measurements of the viscosity and elasticity of a fluid.
Various methods have also been proposed for measuring the characteristics of vessels that contain fluids. Such methods include, for example, measuring the stirring power of an agitator or mixing device within the reactor. U.S. Pat. No. 4,237,261, Kawamura et al., discloses a process for automatically and continuously measuring the stirring power (the externally applied power of a motor) or stirring axis reaction force (the resistance of the polymer to stirring) of at least the final polymerization reactor among all the polymerization reactors, from which a polymer having a limiting viscosity of 0.25 or more is removed. In Kawamura, the polymerization process is controlled by changes in the reactor vacuum. The vacuum in the polymerization reactor is varied based on changes in the stirring power or the stirring axis reaction force so that the stirring power or stirring axis reaction force is kept at a prescribed level whereby the degree of polymerization of the polymer removed from the polymerization reactor is controlled.
U.S. Pat. No. 5,649,449, Algers, discloses a method and apparatus for determining the current or instantaneous operation conditions of a centrifugal pump. In Algers, the radial forces impressed on the impeller shaft by the rotating pump impeller are measured at the shaft-supporting bearing disposed proximate the pump housing and impeller. In a preliminry or test operation of the pump, the radial force measurement is taken at a plurality of volumetric liquid flow rates through the pump so as to develop a relationship between the radial force and liquid flow. Then, during normal operations of the pump, the current or instantaneous radial force is measured at the bearing and this measurement is compared to the previously developed relationship to accurately determine the instantaneous operating flow conditions of the pump by identifying the point along the pump""s characteristic curve, which defines for the pump a relationship between lifting height and volumetric liquid flow, at which the pump is currently operating. In Algers, the unit forces cannot be measured when the pump is empty. Furthermore, Algers does not disclose how to measure the inventory of the equipment to which the pump is attached, only the flow rate through the pump.
With respect to methods for monitoring or controlling the inventory of a fluid in a reactor or a vessel, metering or gear pumps can be incorporated in the supply to and removal of material from such a reactor or vessel and are a means of controlling both flow through the reactor or vessel or inventory within the reactor or vessel. However, metering pumps or gear pumps for example, used with polymer reactors have a mass flow error related to speed, which depends on the viscosity of the polymer being pumped and the pressure that must be developed that prevents speed alone from being a means of precise level or inventory control over an extended continuous operating period.
Conventional inventory monitoring methods in polymer reactors include differential pressure measurement devices that infer a column of liquid of an assumed density to infer a fluid height, flotation or volume displacer devices. Absorption of radiation from a nuclear source in which attenuation of the radiation signal occurs at a detector dependent on the material in the path of the radiation beam may also be used. The accuracy of this method is dependent upon many factors including selection of the source type, construction of the vessel, background radiation intensity and the type of polymeric material being measured. Typically these and other conventional inventory monitoring methods give a relative value which is indicative of the material present only in the exact location of measurement, and consequently such devices do not measure the total amount of inventory present within a reactor or vessel.
In the operation of a continuous polymerization reactor, it is very difficult to measure the total polymer inventory and even more difficult to maintain an accurate inventory over an extended period of time. Consequently, the need for accurate inventory control exists.
Another disadvantage associated with conventional monitoring devices for polymeric reactors is that they require penetration of the vessel shell, jacket, and wall, with insertion of a sensing component into the inner vessel space. Physical insertion of a sensor into a reactor and the use of conventional monitoring devices such as flotation devices, bubble tubes, and nuclear sources makes it difficult, if not impossible, to accurately monitor inventory when the reactor has an agitator which fits close to the walls, when the reactor has complicated internals or when the reactor has a large amount of metal within the reaction space. Additionally, when a reactor has an agitator(s) and is connected to other mechanically rotating devices, vibration makes these systems unreliable for the purposes of inventory control, especially when using weigh cells as the sensor technology.
For example, in the reactor described in U.S. Pat. No. 5,814,282, conventionally known measurement devices do not work. Component insertion is prohibited since the shaft and all of its components are in immediate proximity to the wall of the reactor.
Gear pumps, operating at a controlled speed, are typically used to supply and extract polymer from reactors such as described in U.S. Pat. No. 5,814,282. A disadvantage is that inherent minor errors in the quantitative delivery or extraction of material by such a pump will cause long term variability in the absolute reactor inventory that is unacceptable in the development of polymer properties within the reactor. This is especially important where the design of the reactor does not permit even the use of monitoring devices that provide relative measurements of the inventory.
Therefore, there is a need for a reliable method of monitoring and measuring the absolute inventory of a fluid in a vessel such as a reactor, and there is a further need for a method of controlling the absolute inventory of a fluid in a vessel such as a reactor.
The present invention provides a method for measuring and optionally controlling the absolute inventory of a dynamic fluid in a vessel, particularly a vessel having at least one agitator with a rotatable shaft extending in an essentially horizontal direction. Absolute inventory within the vessel is controlled through use and measurement of a resultant agitator force acting on the agitator in an essentially vertical direction in relation to the shaft of the agitator.
An important aspect of the present invention is the discovery that a relationship exists between the absolute inventory of a vessel and the force acting on the agitator in the essentially vertical direction which can be measured external to the vessel and used to control the vessel inventory over extended periods of continuous operation.
The method of the present invention, therefore, comprises measuring the resultant force acting on the agitator in the essentially vertical direction under a set of operating parameters and at an absolute inventory of fluid to generate a data point. This force measurement is repeated at multiple absolute inventory levels in order to generate a series of data points. The force measurements are preferably taken while holding the non-inventory related operating parameters (temperature, pressure, etc.) constant. The force measurements are taken using a force measurement instrument such as a strain gauge transducer attached in a suitable manner to the agitator shaft, preferably outside of the vessel space. The series of data points are correlated to establish a relationship between the known inventory of the fluid in the vessel and the resultant force acting on the agitator in the essentially vertical direction. The resultant force acting on the agitator in the essentially vertical direction then acts as the controlled process variable to maintain a desired absolute inventory of fluid according the relationship(s) that was established. The operating parameters of the reactor and components attached thereto, for example, the speed of a metering gear pump, are then adjusted through a series of control loop functions to maintain constant the resultant force acting on the agitator in essentially the vertical direction that corresponds to the desired absolute inventory.
In a preferred embodiment of the present invention, the fluid is a polymer. Particularly preferred polymers include polyesters, copolyesters, polyarylates, copolyarylates, polyamides, and copolyamides. The polymer may be present in the vessel in an amount ranging from about 10 to 70 percent of the available operating volume of the vessel.
In another embodiment of the present invention, the vessel is a reactor especially suitable for polycondensation reactions in which a volatile component is removed and a polymeric material of high viscosity is produced from a polymeric material of lower viscosity while remaining in a liquid state within the reactor. In another embodiment of the present invention, the reactor has two rotatable shafts. In another embodiment of the present invention, the two shafts counter rotate.
An advantage of the present invention is that the total absolute inventory of the fluid within the vessel is known at any moment in time. Acting as an input to a process control loop, for example, as in a form of feedback control, the amount of fluid in the vessel is adjusted, if necessary, to provide a constant measured force acting on the agitator in the essentially vertical direction, which in turn provides for a constant absolute inventory in the vessel. Therefore, an advantage of this method over the conventional monitoring methods is that at any moment in time the total absolute amount of the fluid in the vessel is known as opposed to the amount of the fluid at just a localized point.
Therefore, it is a principal object of the present invention to provide a reliable method for controlling the absolute inventory of a fluid in a vessel such as a reactor.
Other features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that that the drawings are not drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.